It is known that, under suitable pressure and temperature conditions, urea can be formed from ammonia and carbon dioxide by means of a two-step reaction. The ammonia and carbon dioxide are first reacted to give an ammonium carbamate intermediate product according to the equation ##STR1## Reaction (1) is highly exothermic and proceeds rapidly and to completion if the reaction pressure and temperature are sufficiently high -- i.e. above 90 atmospheres and 155.degree. C, respectively.
Urea is formed from the ammonium carbamate by dehydration according to the equation ##STR2## Reaction 2, which proceeds endothermically, is an equilibrium reaction wherein the chemical equilibrium is established comparatively slowly, and is shifted towards the right as the pressure and the urea:water ratio are increased.
The prior art conventionally conducted the urea synthesis at a temperature of 160.degree. - 200.degree. C and a pressure of 100 - 250 atmospheres. The urea synthesis solution leaving the urea synthesis zone normally contains unconverted ammonium carbamate. The ammonium carbamate is normally recovered from the urea synthesis solution by decomposition of the ammonium carbamate into gaseous ammonia and carbon dioxide. The decomposition is normally conducted in a number of pressure stages, and the resulting gaseous mixtures of ammonia and carbon dioxide, together with any excess ammonia and the equilibrium amount of water vapor, are separated from the urea synthesis solution. These separated gas mixtures are generally recycled after condensation and/or absorption in water or in an aqueous solution in the above-mentioned pressure stages. Following this recycle process, it will be appreciated that water is necessarily returned to the urea synthesis zone. However, the presence of such water has an adverse effect upon the conversion of ammonium carbamate into urea, and it is therefore desirable to restrict the amount of recycled water as much as possible.
U.S. Pat. No. 3,356,723, issued Dec. 5, 1967, discloses a process for preparing urea wherein the decomposition of the unconverted ammonium carbamate, and the separation of the gaseous mixture from the urea synthesis solution is effected by heating the urea synthesis solution at elevated pressures while a carbon dioxide stripping medium is passed through the urea synthesis solution. The gaseous mixture which is separated from the urea synthesis solution is condensed under elevated pressures with the condensation temperature increasing as the pressure at which the condensation is conducted increases. The heat of condensation may be recovered at a higher temperature level than in earlier processes, and consequently this heat can be used for the production of higher temperature steam. The solidification point of ammonium carbamate is 153.degree. C and in operation at temperatures below this level, the amount of water required to keep the ammonium carbamate in solution (which water is then returned to the urea synthesis zone) is less as the temperature is increased. Preferably, the condensation is conducted in excess of 153.degree. C, as no additional water must then be added. Also, another reason for using the highest possible condensation temperature is that a higher conversion of ammonium carbamate to urea is obtained at higher temperatures. The condensation pressure, however, is limited by the maximum permissible pressure for the stripping operation, since compression of the gas mixture separated in the stripping operation is less undesirable (because of the risk of solid ammonium carbamate depositing in the compressor and conduit lines, and also because of the high cost of compression). The highest permissible stripping pressure is in turn directly related to the temperature level at which the heat of decomposition may be supplied without causing undesirable hydrolysis of urea and biuret formation. The highest permissible stripping pressure is considerably lower than the optimum urea synthesis pressure.
The aforementioned patent suggests, e.g. see FIG. 1 thereof, that the ammonium carbamate solution formed in the condensation zone be pumped to the urea synthesis zone at a pressure which is favorable to the conversion of ammonium carbamate into urea. In this process, all of the heat that is released in the formation of the ammonium carbamate solution in the ammonium carbamate condensation zone is removed by cooling water, and consequently such heat is available only at a low temperature level and thus has limited use. The cooling water is also used to remove heat that is released in the formation of ammonium carbamate from ammonia and carbon dioxide according to Reaction (1). However, the process requires the addition of heat from outside sources for the conversion of ammonium carbamate into urea and for the stripping treatment of the urea synthesis solution, with such heat normally being supplied through high pressure steam (at 15 - 25 atmospheres pressure). It will be appreciated, therefore, that the process of this patent has the drawback that most of the potentially useful heat released during the process is converted into heat having a low economic value -- i.e. steam degradation occurs.